I want to sell my pocket 2, but I can't remove it from my account. Will that be a problem for new user that I have a binding to this device?

No problem at all.
This is how DJI works.
New users are 'added'.

Hi there. Thanks for reaching out. As mentioned in the comment above, it will not affect the new user's usage. They can just log in to the DJI Mimo with their account. If they log in to the app using their own account, it will be displayed as "******." We hope this clarifies your concern.